1. Field of the Invention
The present invention generally relates to the field of semiconductor processing. More particularly, the present invention relates to methods and apparatus for thermally processing a semiconductor substrate.
2. Background
The fabrication of integrated circuits from silicon or other wafers involves many steps of depositing layers, photo lithographically patterning the layers, and etching the patterned layers. Ion implantation is used to dope active regions in the semiconductive silicon. The fabrication sequence also includes thermal annealing of the wafers for many uses including curing implant damage and activating the dopants, crystallization, thermal oxidation and nitridation, silicidation, chemical vapor deposition, vapor phase doping, thermal cleaning, and other reasons. Although annealing in early stages of silicon technology typically involved heating multiple wafers for long periods in an annealing oven, rapid thermal processing, (RTP) has been increasingly used to satisfy the ever more stringent requirements for ever smaller circuit features. RTP is typically performed in single-wafer chambers by irradiating a wafer with light from an array of high-intensity lamps directed at the front face of the wafer on which the integrated circuits are being formed. The radiation is at least partially absorbed by the wafer and quickly heats it to a desired high temperature, for example above 600° C., or in some applications, above 1000° C. The radiant heating can be quickly turned on and off to controllably and uniformly heat the wafer over a relatively short period, for example, of a minute or less, or even a few seconds. RTP chambers are capable of uniformly heating a wafer at rates of about 50° C./second and higher, for example, at rates of 100°-150° C./second, and 200°-400° C./second. Typical ramp-down (cooling) rates in RTP chambers are in the range of 80-150° C./second. Some processes performed in RTP chambers require variations in temperature across the substrate of less than a few degrees Celsius.
Since rapid thermal processing works on a single semiconductor each time, optimal heating and cooling means are necessary for optimal RTP performance. It is desirable to optimize substrate temperature uniformity during thermal processing of the substrate. Temperature uniformity provides uniform process variables on the substrate (e.g. layer thickness, resistivity, etch depth) for temperature activated steps such as film deposition, oxide growth and etching. In addition, substrate temperature uniformity is necessary to prevent thermal stress-induced substrate damage such as warpage, defect generation and slip. For example, at 1150° C., a center to edge temperature difference on a four-inch silicon wafer of approximately 5° C. can induce dislocation formation and slip. Temperature gradients may also be induced by other sources. For example, a substrate may have non-uniform emissivity because of spatial modifications to surface areas or volumes of the substrate. These modifications may include films that have been patterned by photolithography or locally doped regions, such as buried layers for bipolar transistors. In addition, substrate temperature gradients may be induced by localized gas cooling or heating effects related to processing chamber design as well as non-uniform endothermic or exothermic reactions that may occur on the substrate surface during processing. It would be desirable to provide RTP chambers that provide improved temperature uniformity.
As noted above, RTP usually requires a substantially uniform temperature profile across the substrate. In the state of the art process, the temperature uniformity may be improved by controlling heat sources, such as a laser, an array of lamps, configured to heat the substrate on the front side while a reflective surface on the back side reflects heat back to the substrate. Emissivity measurement and compensation methodology have been used to improve the temperature gradient across the substrate.
As the semiconductor industry develops, the requirement for temperature uniformity during a RTP also increases. In some processes, it is important to have substantially small temperature gradient from about 2 mm inside the edge of the substrate. Particularly, it may be necessary to heat a substrate at a temperature between about 200° C. to about 1350° C. with a temperature deviation of about 1° C. to 1.5° C. The state of the art RTP systems have difficulties to reach this kind of uniformity, especially near the edge of the substrate. In a RTP system, an edge ring is usually used to support a substrate near the periphery. The edge ring and the substrate overlap producing a complicated heating situation near the edge of the substrate. In one aspect, the substrate may have different thermal properties near the edge. This is mostly pronounced for a patterned substrate, or for a silicon-on insulator- (SOI) substrate. In another aspect, the substrate and the edge ring overlap near the edge, it is difficult to achieve uniform temperature profile near the edge by measuring and adjusting the temperature of the substrate alone. Depending on the edge ring's thermal properties relative to the substrate's thermal and optical properties, the temperature profile of a substrate is generally either edge high or edge low.
FIG. 1 schematically illustrates two types of common temperature profiles of a substrate processed in a RTP chamber. The vertical axis denotes measured temperatures on a substrate. The horizontal axis denotes the distance from the edge of the substrate. Profile 1 is an edge high profile where the edge of the substrate has the highest temperature measurement. Profile 1 is an edge low profile where the edge of the substrate has the lowest temperature measurement. It is difficult to remove temperature deviation near the edge of the substrate in the state of the art RTP systems.
Residual heat remains in the RTP chamber after processing a wafer. One of the parts that is adversely affected is the wafer support (a.k.a. edge ring). To avoid breaking/warping wafers and improving wafer uniformity due to a hot edge ring, the wafer must be heated before it is placed on the edge ring, causing a decrease in the overall throughput. The wafer must be heated close to the edge ring temperature to avoid breaking or warping. Conversely, the edge ring may be cooled to achieve the same desired effect, which is to minimize the difference in temperature between the edge ring and the wafer.
Another benefit to controlling the edge ring temperature is observed during wafer processing. The edge ring temperature may be heated or cooled to minimize the difference in temperature between the edge ring and the wafer. This control would allow for better management of the discontinuity caused by the overlap of the wafer and the edge ring. This cannot be done as well with the current RTP lamp assembly because the radiation covers an area larger than the edge ring. Heat sources can be adapted to radiate substantially only the edge ring so the edge ring heating can be varied to accommodate its changing radiation heat characteristics relative to the wafer during the entire process cycle.
Therefore, there is a need for apparatus and methods used in RTP for improved temperature uniformity.